The proposed studies will examine two related questions in two different population. In a general population, what are the dose-response effects between physical activity and fitness to symptoms of depression and psychological well-being? In participants with diagnosed mild to moderate major depressive disorder (MDD), what is the efficacy of exercise as a treatment? The rationale for these studies is as follows. A recent consensus statement documents that depression is undertreated in the community for a variety of reasons including a lack of effective alternative treatments. Exercise, a possible alternative treatment, has been shown to be related to fewer depressive symptoms in observational studies and has been demonstrated to be as efficacious as psychotherapy in patients with mild to moderate depression or an effective adjunct in more severely depressed patients in short term experimental an quasi-experimental studies. However, the Clinical Practice Guidelines for Treatment of Major Depression do not list exercise as a possible treatment or an adjunction to treatment because evidence for the efficacy for exercise treatment does not yet come from randomized controlled clinical trials. A recent review calls for observational studies to examine the dose-response relation in both large cross-sectional and longitudinal analyses that have adequate measures of depression and physical activity and fitness. In addition, randomized clinical studies need to use adequate controls and doses of activity to test the dose-response relation of exercise as an efficacious treatment for depression. The proposed studies will address these deficiencies. The primary research hypotheses are: (1) there are dose-response relations between exercise and depressive symptoms and psychological well-being; and (2) exercise is an efficacious treatment for mild to moderate MDD. For the first hypothesis, the population-based approach will use a subset of the Aerobics Center Longitudinal Study data from clinic visits (n=8,345) and mail-back surveys (n=13,627 in 1990 and n=13,329 in 1995). This phase of cross-sectional and longitudinal analyses will start immediately while planning begins for the second phase. The second phase, a randomized clinical treatment study, will examine the efficacy and dose-response relation of exercise in treating mild to moderate depression in 140 adults 20-40 years old. The experimental study is a 2 x 2 design with an exercise placebo control. All treatments will be 3 months in duration with 3 months of additional follow-up. We believe these studies will provide basic information needed for scientific understanding of the exercise depression relation and will provide evidence of whether exercise is an effective alternative treatment for mild to moderate levels of MOD.